High in the Alps of Graubünden canton in Switzerland, a road was being constructed to link the remote village of Schuders to the town of Schiers in the valley below. The route involved a crossing of the deep Salgina gorge and a bridge was needed. In July 1928, Maillart started work on a design for this bridge, and the Ingenieurbureau Maillart bid named trusted contractor Florian Prader (1883-1946), with whom he had worked many times. In October it became clear that the bid had won the tender.

The Salginatobel Bridge (Salginatobelbrücke) would be completed in 1930 and it is probably Maillart's best-known structure. Its design epitomises the efficient, economical and elegant structures that are synonymous with his name. It is now a Swiss heritage site of national significance, and in 1991 was designated an International Historic Civil Engineering Landmark. In 2001, the British trade journal Bridge – Design and Engineering voted it "the most beautiful bridge of the century".

In the meantime, 1929 was a busy year for Maillart, despite a global economic crisis and economic recession in many countries, including Switzerland. His year began with preparations for the wedding of his daughter Marie Claire (1906-2009). In February, after a brief engagement, she married civil engineer Eduard Henri Blumer (1901-80) in the Glarus Alps. They departed at once for Serang-Djaja in South Sumatra, Dutch East Indies (now Indonesia), where Blumer was working for oil company Royal Dutch Shell (now Shell).

Then the Salginatobel Bridge project started. The site is challenging, requiring a leap of 90m across a gorge too high for intermediate support to be possible. Maillart's solution is a concrete three-hinge, open box arch that springs from the living rock on the east side to a concrete abutment on the west side.

As ever, he preferred to find the form by plotting the effects of loading and making adjustments, rather than relying on calculations alone. The arch is designed to be in compression as much possible. However, the beauty of the form was no less important to him.

Construction began with an intricate double fan lattice of timber centring, designed by Richard Coray (1869-1946), strong enough to support the casting of the reinforced open box of the arch. As with previous bridges, once the arch was fully hardened, it became the support for the casting of the transverse walls in the trough, and for the deck above. All the casting was carried out in early 1930 and took just three months.

Maillart tackled another substantial project in 1929. While the Salginatobel Bridge was to gather praise, the Sihlholzli Sports Centre (Turnhalle Sihlhölzli) in Zürich was to spark controversy after its completion in 1932. He took it on after the death of its original design engineer, Dr Ernst Suter (c.1884-1929).

In March 1933, details were published in Schweizerische Bauzeitung. Unfortunately, a draughting error in the roof truss reinforcement on the construction drawings had resulted in its central concrete hanger containing far less steel than anticipated. Maillart maintained that the roof was structurally sound and proposed a load test to prove it, but Professor Max Ritter (1884-1946, not to be confused with Karl Wilhelm Ritter) of the Eidgenössischen Technischen Hochschule (Swiss Federal Institute of Technology) in Zürich intervened.

Ritter and Maillart had disagreed before over methods of structural analysis, with Maillart in favour of determining results through physical testing and graphical assessment and Ritter championing theoretical calculations. Ritter was involved in the preparation of a new code for reinforced concrete, approved in June 1933 by the Schweizerische Ingenieur- und Architektenverein (Swiss Society of Engineers & Architects) and published in May 1935.

Maillart's April 1933 report on the sports centre roof showed that it complied with the Swiss steel code but in February 1934 Ritter argued it did not satisfy the concrete code. In September 1934, Maillart installed additional horizontal struts to bypass the central roof hangers, though the dispute dragged on until May 1935.

At the beginning of 1930, before the sports hall was finished, Maillart tackled the challenge of engineering a 30m span rail bridge on which the tracks and deck followed a curve in plan. His successful design for the Landquart Bridge (Landquartbrücke) at Klosters used 60% less concrete than the cantonal design. Also completed in 1930 was the concrete block arch Lorraine Bridge (Lorrainebrücke) over the River Aare in Berne canton.

Maillart designed 24 bridges in the first half of the 1930s, though seven are unbuilt as he failed to win the tenders. He also designed a number of buildings, often working collaboratively with architects and other engineers. Projects included a tannin factory at Maroggia in Ticino canton, the Crédit Suisse bank building in Geneva, two industrial buildings at Vernier in Geneva canton, two more industrial buildings, a post office and two schools in Zürich, and a factory at Gerlafingen in Solothurn canton.

By 1931, his reputation was such that he had become sought after as a speaker and engineering consultant. His design ethos was summarised in the Schweizerische Bauzeitung article "Masse oder Qualität im Betonbau?" (Mass or quality in concrete structures?), published on 19th September 1931, in which he urged designers to oppose massive structures and use high-quality materials sparingly.

The 1930s was a period of wordlwide economic downtown. So far, Ingenieurbureau Maillart had not been affected too much. Its three offices in Switzerland remained busy. Maillart himself was based in Geneva, where he had an apartment in the office there. He had developed the habit of visiting each office every week.

The smallest office was in Berne, run by Ernst Stettler with one draughtsman. In March 1930, architect Hans Kruck, son of Gustav Kruck (1875-1934) who had been involved in the Wägital scheme that included three of Maillart's bridges, joined the Zürich staff. In early October 1931, the Zürich office relocated to Bleicherweg, overlooking the northern end of Lake Zurich, and engineer Marcel Francis Fornerod (1903-2004) joined the team. In May 1931, Adolph Zarn, Maillart's business partner for almost 30 years, left the Geneva office.

Five small bridges were built in Berne canton during 1931 — a footbridge over the Triftwasser brook near Gadmen and four deck stiffened arch bridges, with spans of 20-30m. The Hombach Bridge (Hombachbrücke) and Luterstalden Bridge (Luterstaldenbrücke) are in Schangnau, and the Ladholz Footbridge (Ladholzbrücke) and Engstligen Bridge below Adelboden (Spitalbrücke) are southwest of Frutigen.

On 15th October, Maillart was returning from a site visit advising on the reconstruction of a bridge in Adelboden when he was hit by a speeding car near Heimberg. Two weeks later, he was diagnosed with a fractured ankle and recuperated in bed, cared for by his sister Rosa's maid Amélie. He devoted the time to developing design ideas.

By the end of 1931, he was mobile again but in constant pain. On 6th February 1932, he celebrated his 60th birthday and described himself as an "old man" in a letter to Marie Claire in the East Indies. On 26th May, his mother Bertha passed away, aged 89. In June, he consulted Rosa's doctor who found that he had internal injuries, to his kidneys or liver, sustained in the previous October's car accident.

That year, three more bridges were completed in Berne canton — the deck-stiffened arches of Bohlbach Bridge (Bohlbachbrücke) and Traubach Bridge (Traubachbrücke) at Habkern, and the 82m span three-hinge arch of Rossgraben Bridge (Rossgrabenbrücke), which was constructed in just three months.

The shrinking Swiss economy was now making itself felt at Ingenieurbureau Maillart. There were no local commissions for the Geneva office and the staff began to work on some of the Berne and Zürich projects. Maillart was living in a two-room rented apartment in the Schwarzen Bären (Black Bear) hotel off Parade Platz in central Zürich.

In Germany, the Nazi Party was steadily gaining power and in September Hermann Göring (1893-1946) was elected President of the Reichstag. On 30th January 1933, President Paul von Hindenburg (1847-1934) reluctantly appointed Adolf Hitler (1889-1945) as German Chancellor. After Hindenburg's death, Hitler became Führer und Reichskanzler (Leader and Reich Chancellor) — in effect, dictator.

Switzerland is sandwiched between Germany to the north and Italy to the south, and Hitler and Italy's fascist leader, Benito Mussolini (1883-1945), posed a perceived threat to the country's independence. Socialists, trade unions and liberal employers co-operated to stand against fascism, and Jews and refugees of all kinds poured into Switzerland. In 1934, the Federal Act on Banks & Savings Banks was passed, safeguarding the famous secrecy of Swiss banking and ensuring the privacy of its clients, including those fleeing the Nazi regime.

The grim political situation was lightened for Maillart by reunions with his three children. In August 1933, he travelled for the first time to Britain to visit his sons, Edmond (1902-62) in London and René (1909-76) on the island of Jersey. In November, Marie Claire, her husband Eduard and Maillart's granddaughter Marie-Claire (b.1930) arrived in Zürich from the East Indies. Eduard returned to work after six months but his wife and daughter remained until late October 1934.

Four bridges were constructed in 1933-4, including the three-hinge hollow box arches of Thur Bridge (Thurbrücke) at Felsegg in St Gallen canton and the Aare Bridge (Aarebrücke) at Innertkirchen in Berne canton, and the deck-stiffened arch of Töss Footbridge (Töss Fussgängersteg) at Winterthur in Zürich canton.

However, the most notable of the four is the Schwandbach Bridge (Schwandbachbrücke), not far from the Rossgraben Bridge. Its stiffened concrete arch spans 37.4m and, though the deck follows an elliptical curve in plan, the arch is straight. On the west (concave) side of the bridge, the arch widens to support the smooth sweep of the deck, resulting in tapered transverse support walls.

In 1935, Maillart took his concrete bridge solutions in a different direction. For small spans or low clearances, deck and parapet beams together worked effectively without an arch and eliminated the cost of centering. The first two such continuous concrete beam bridges were Birs Railway Bridge (Birs Eisenbahnbrücke) at Liesberg in Basel-Landschaft canton and Huttwil road bridge (Huttwil Strassenüberführung) over the Huttwil-Wolhusen railway in Berne canton.

By March 1935, scarcity of work had reduced the Zürich office to just three people — chief engineer Alois Keller, Kruck and a draughtsman. Maillart had had to make Fornerod redundant at the end of October 1933 (Fornerod would move to New York in 1937). In November 1935, Keller was let go. Presumably Kruck and the draughtsman remained at work. Maillart continued to divide his time between the Geneva, Berne and Zürich offices.

The second half of the 1930s, and the last years of Maillart's life, would lead to another world war, illness, bereavement and another visit from his daughter. Despite the diffculties, he was as full of ideas as ever and would design 14 more bridges and some unusual structures.